问题如下：
An analyst is trying to calculate the returns and standard deviation of different asset classes based on their historical data. Which of the following asset class will have the highest return and standard deviation?
选项：
A.Small-cap stocks.
B.Long-term corporate bonds.
C.Large-cap stocks.
解释：
A is correct.
Based on historical data, small-cap stocks have the highest annual return and standard deviation of return among the asset classes.
请问为啥小公司的股票有很高的标准差呢，由哪个公式来的呀，谢谢
